Bacterial isolates obtained from subgingival sites recognize and interact with other oral bacteria belonging to different genera to form large, visibly detectable coaggregates. Among both fresh isolates and stock culture strains, most Streptococcus sanguis strains coaggregate with most Actinomyces viscosus and A. naeslundii strains. The same nonrandom and highly specific patterns of coaggregation were observed with all strains independent of their source and including those obtained from the same site. These cell-to-cell interactions are most likely mediated by complementary surface components composed of a lectin on one cell type and a carbohydrate receptor on the other cell type. One component on A. viscosus probably serves a dual function as a bacteriophage receptor and a coaggregation mediator. The parent strain possesses both functions whereas bacteriophage resistant mutants have lost both. Another likely example of a dual function surface component was found on S. sanguis which mediates coaggregation with A. viscosus and A. naeslundii with the same structure as used to coaggregate with Bacteroides loescheii. Interactions of this type are hallmarks of the ecological importance of coaggregation in the colonization of bacteria on the tooth surface and the subsequent development of periodontal disease.